There is an effort to utilize the potential in this country for producing coal as a primary source of fuel. There are problems in adapting the fluidized bed combustor to furnaces of utility-sized boilers. Industry is going through a new cycle where boilers must be adapted to utilize a fluidized bed for burning coal. The fluidized bed combustor has experimentally achieved acceptable levels of combustion efficiency and sulfur capture.
In obtaining full benefit from the effects of freeboard residence time and recycle, a "ranch style" fluidized bed was chosen. The name is derived from the arrangement of the fluidized bed on a single elevation. It was selected as the best from eight configurations. The eight concepts were essentially variations of three basic types: the ranch, a single vertical stacked bed arrangement, and a multiple stacked bed design.
Limitations in forming the fluid bed cells in the stacked design result in additional waterwall surface area; thus, the freeboard temperature tends to drop rapidly, reducing the residence time at useful temperatures to below two seconds. In the ranch design, a large freeboard is provided above the bed. The residence time in the freeboard was found to be three seconds for a 200-MW boiler, increasing to 5 seconds in an 800 MW unit. The relationship between freeboard volume and heat-absorbing wall surface is such that the temperature in the freeboard remains high enough to support additional completion of the combustion and sulfur capture reactions.
Elutriated fines are captured by a dust collector and recycled through the bed to increase the residence time of solids in the system. The freeboard residence time of 3 to 5 seconds enables additional reaction between solids and gases above the bed. The release of energy above the bed produces temperatures as high as 1700.degree. F. which aids in the burn-out of carbon and in reduction reactions between carbon and oxides of nitrogen which minimize NOx emissions. The long freeboard residence time permits the effective use of overfire air. While the overall excess air is 20%, the bed can be operated at lower excess air, producing conditions favoring NOx reduction in the bed and in the lower section of the freeboard. This excess air level must be sufficient to prevent reducing conditions that could promote corrosion. The remaining excess air is injected into the freeboard to promote gas mixing and improve carbon burn-up.
In addition to using a design approach well suited to the fluidized bed process, the ranch design provides advantages in the mechanical design of supporting auxiliary equipment. With the bed on a single elevation, fuel and bed drain piping and air ducting are easily routed to the bottom of the boiler. Maintenance is simplified with accessibility near ground level to the major auxiliary equipment for fluidized bed operation.
A minimum of structural steel is used for the boiler in the ranch configuration. A fluidized bed, which contains a large inventory of material, is bottom supported by ground level steel structure. These substantial loads, as well as the dynamic forces of fluidization, are carried very efficiently. The relatively light freeboard structure and convective sections require top support in the conventional manner. However, the top-supported loads are significantly less than even in a conventional boiler. The savings in structural steel represent a major cost reduction over stacked bed designs. With both top and bottom support of the boiler, the problem of compensating for thermal expansion and contraction requires a solution not heretofore evident.